The present invention relates to the control of low fluid flows.
More particularly, the invention provides a variable flow regulator able to provide accurate flow rates, at low flow and ultra low flow rates, typically as required in medical and laboratory applications.
In hospitals and clinics medicines and other fluids are administered to patients for a variety of purposes. Most common is the administration of infusions to patients who are dehydrated or not eating and drinking for a variety of reasons. Flow rates can be adjusted according to the needs of the patient, typically in the range 100-500 ml per hour.
Medicines not taken by mouth can be added to an infusion, or are injected by the use of a syringe having a hollow needle, the contents of a 5 to 25 ml ampoule being received by the patient in 2-5 seconds.
These methods cannot be utilized where medical fluids are to be received by the patient at controlled and very slow feed rates, typically under 10 ml per hour,
Such time-extended flow requires a source of power other than the finger pressure of a medical attendant on a syringe plunger. The power source can be gravity or an elastic element, in combination with a flow regulator, or an electrically driven pump can be used with or without a flow regulator.
Certain patients receive medication, for example insulin, from devices carried on their body in a belt pouch or clothing pocket. Portable devices, normally powered by electric batteries, can serve this purpose.
Adjustable flow, mains electricity-driven metering pumps are used in industrial and chemical applications, and for dispensing, for example, liquid fertilizers, chlorine and fluoride. A positive displacement peristaltic pump of this type marketed under the MECOMATIC trade name can be adjusted down to 0.25 gallons per day. Although the tubing can be readily changed, this pump weighs 9.5 lb. and is obviously not disposable, and such pumps are not suitable for medical applications requiring extremely slow flow rates.
In U.S. Pat. No. 4,544,369 Skakoon et al disclose a battery-operated miniature syringe infusion pump. The device includes a battery, a DC motor, gearing, electronic controls, a lead screw, a force-sensing system, indicators, and alarms and is light enough to be mounted on an IV pole. The main drawbacks are high initial costs, lack of portability and the need for batteries. U.S. Pat. No. 5,034,004 to Crankshaw is generally similar to the Skakoon specification but involves even more mechanical and electronic components.
A less complex arrangement is disclosed in U.S. Pat. No. 4,976,696, which specification includes control means for regulating the movement of a driver against the syringe plunger. It is however uncertain whether the very slow flow rates which are a main feature of the present invention can be handled merely by controlling plunger movement, and without the use of a further fluid resistor.
It is therefore one of the objects of the present invention to obviate the disadvantages of prior art flow regulators and to provide a device which can reliably deliver low flows and very low flows.
It is a further object of the present invention to provide a regulator which can be adjusted within a wide range in its capacity.
Yet a further object of the present invention is to provide a self-powered regulator for delivering fluids at very low flow rates.
The present invention achieves the above objects by providing a wide-range flow regulator for low fluid flows, comprising a housing divided into a first and a second chamber by means of an elastic diaphragm, said first chamber having an inlet for a fluid, and an outlet connected to a labyrinth path.
The second chamber has an inlet connected to an outlet of said labyrinth path, the outlet of said second chamber being an inlet at an extremity of a discharge tube, the discharge tube inlet being positionable at a short distance from the diaphragm.
Means are provided to vary said distance to achieve fine adjustment of the flow rate, the discharge tube inlet being sealed by the diaphragm when said diaphragm flexes in response to pressure drop in the second chamber. The labyrinth path connects the first chamber to the second chamber to allow the liquid to flow under controlled pressure drop from the first chamber to the second chamber and so to intermittently raise pressure in the second chamber to temporarily separate the diaphragm from the discharge tube inlet. This allows liquid to enter and traverse the discharge tube for use of said fluid.
Means are provided to selectively short-circuit at least a portion of the labyrinth path to vary the range of the fluid flow rate.
In a preferred embodiment of the present invention there is provided a flow regulator wherein the inlet of said first chamber is connectable to receive fluid by gravitational means.
In a most preferred embodiment of the present invention there is provided a flow regulator wherein said inlet of the first chamber is connectable to receive fluid from a syringe.
Yet further embodiments of the invention will be described hereinafter
In U.S. Pat. No. 5,421,363 Bron describes and claims an adjustable-rate flow regulator intended for use between an infusion bag and a tube leading to a patient. The device relies entirely on changing the used length of the labyrinth in order to effect changes in the flow rate. Changes in the length of the labyrinth are effective for coarse adjustment, but less effective for fine adjustment. Total reliance on labyrinth length will not ensure accuracy, as slight viscosity changes in the fluid, for example as result of temperature variations, will result in substantial flow variation.
A further disadvantage of total reliance on the labyrinth lies in the fact that the covered range is limited. Tests carried out with plastic drip irrigation tubing have shown that where the labyrinth is long, substantial labyrinth length changes result in only marginal changes in flow rates.
Furthermore, the performance of the Bron device is dependent on the stiffness of the flexible diaphragm. Such stiffness can vary widely with conditions of age, humidity and exposure to chemical attack.
In contradistinction thereto, the present invention, in addition to labyrinth length adjustment, also provides means for fine adjustment of flow rates, by providing means for adjusting the distance between the diaphragm and the inlet of the discharge tube. Consequently the device of the present invention can be used over a very wide range, typically from 1 to 200 ml per hour. The stiffness of the diaphragm has little effect on performance, as a spring provides the force needed to press the diaphragm into contact with the regulator inlet
It will also be realized that the novel regulator of the present invention can be mass produced by plastic injection molding of its individual parts and that assembly of the components can be effected in very little timexe2x80x94easily in less than a minute. Consequently the device can be marketed for disposable use, as is preferred for medical applications.
The invention will now be described further with reference to the accompanying drawings, which represent by example preferred embodiments of the invention. Structural details are shown only as far as necessary for a fundamental understanding thereof. The described examples, together with the drawings, will make apparent to those skilled in the art how further forms of the invention may be realized.